Re: [PATCH 2/3] gpiolib: use kref in gpio_desc

[Bartosz Golaszewski <brgl@bgdev.pl>]

pt., 13 mar 2020 o 16:04 Bartosz Golaszewski <brgl@bgdev.pl> napisał(a):
>
> pt., 13 mar 2020 o 09:44 Linus Walleij <linus.walleij@linaro.org> napisał(a):
> >
> > On Thu, Mar 12, 2020 at 7:25 PM Bartosz Golaszewski
> > <bgolaszewski@baylibre.com> wrote:
> >
> > > I believe this is not correct. The resources managed by devres are
> > > released when the device is detached from a driver, not when the
> > > device's reference count goes to 0. When the latter happens, the
> > > device's specific (or its device_type's) release callback is called -
> > > for gpiolib this is gpiodevice_release().
> >
> > Yeah you're right, I even point that out in my second letter :/
> >
> > It's a bit of confusion for everyone (or it's just me).
> >
>
> No, it is confusing and I only recently understood it while trying to
> fix a memory leak in nvmem.
>
> > > The kref inside struct device will not go down to zero until you call
> > > device_del() (if you previously called device_add() that is which
> > > increases the reference count by a couple points). But what I'm
> > > thinking about is making the call to device_del() depend not on the
> > > call to gpiochip_remove() but on the kref on the gpio device going
> > > down to zero. As for the protection against module removal - this
> > > should be handled by module_get/put().
> >
> > Right. At the end of gpiochip_remove():
> >
> >    cdev_device_del(&gdev->chrdev, &gdev->dev);
> >    put_device(&gdev->dev);
> >
> > That last put_device() should in best case bring the refcount
> > to zero.
> >
> > So the actual way we lifecycle GPIO chips is managed
> > resources using only devm_* but the reference count does work
> > too: reference count should normally land at zero since the
> > gpiochip_remove() call is ended with a call to
> > put_device() and that should (ideally) bring it to zero.
> >
> > It's just that this doesn't really trigger anything.
> >
>
> Not necessarily - if the new kref for GPIO device would be detached
> from device reference counting and what it would trigger at release is
> this:
>
>    cdev_device_del(&gdev->chrdev, &gdev->dev);
>    put_device(&gdev->dev);
>
> Or to be even more clear: "getting" the gpiodevice would not be the
> same as "getting" a device - in fact only when the gpiodevice kref
> goes down to 0 do we put the reference to the device object.
>
> > I think there is no way out of the fact that we have to
> > forcefully remove the gpio_chip when devm_* destructors
> > kicks in: the driver is indeed getting removed at that
> > point.
> >
>
> There does seem to be a way around that though: the clock framework
> does it by creating a clock "core" object which is reference counted
> and if the provider is removed while consumers still hold references
> to it, then it does a couple things to "numb" the provider (as you
> nicely put it) like replacing all ops callbacks with NULL pointers but
> keeps the structure alive until the consumers also give up all their
> references.
>
> That being said: I'm not saying this is necessary or even useful. I
> started the discussion because I was under the impression I wasn't
> clear enough when writing about reference counting for descriptors. If
> nobody complains about the current implementation then let's not fix
> something that's not broken.
>
> Bartosz
>
> > In gpiochip_remove() we "numb" the chip so that any
> > gpio_desc:s currently in use will just fail silently and not crash,
> > since they are not backed by a driver any more. The descs
> > stay around until the consumer releases them, but if we probe the
> > same GPIO device again they will certainly not re-attach or
> > something.
> >
> > Arguably it is a bit of policy. Would it make more sense to
> > have rmmod fail if the kref inside gdev->dev->kobj->kref
> > is != 1? I suppose that is what things like storage
> > drivers pretty much have to do.
> >
> > The problem with that is that as soon as you have a consumer
> > that is compiled into the kernel it makes it impossible to
> > remove the gpio driver with rmmod.
> >
> > I really needed to refresh this a bit, so the above is maybe
> > a bit therapeutic.
> >
> > I don't really see how we could do things differently without
> > creating some other problem though.
> >
> > Yours,
> > Linus Walleij

Hi Linus,

what is your decision on this? Because if we don't merge this, then we
need to make sure nvmem doesn't call gpiod_put() for descriptors it
didn't obtain itself and we should probably fix it this week.

Bart